Fair Value of Financial Instruments - Schedule of Significant Unobservable Inputs Used to Calculate Level 3 Fair Value of Embedded Derivatives within Policyholder Contract Deposits (Detail) - Policyholder Contract Deposits - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Indexed Annuities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative financial liabilities	$ 656.4	$ 592.8
Indexed Life
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative financial liabilities	$ 12.1	$ 3.3
Minimum | Level 3 | Indexed Annuities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable input, lapse rate	1.00%	1.00%
Unobservable input, mortality multiplier	90.00%	90.00%
Unobservable input, equity volatility	12.00%	19.00%
Minimum | Level 3 | Indexed Life
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable input, equity volatility	12.00%	19.00%
Maximum | Level 3 | Indexed Annuities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable input, lapse rate	70.00%	70.00%
Unobservable input, mortality multiplier	100.00%	100.00%
Unobservable input, equity volatility	25.00%	26.00%
Maximum | Level 3 | Indexed Life
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable input, equity volatility	25.00%	26.00%